Of each series of twelve ribs only the upper seven have their cartilages directly connected with the sternum : the next three cartilages each join the one immediately above, so only reach the sternum indirectly, and the two last ribs have their short cartilaginous ends terminating without such junction, lying in the muscles of the body wall.
The upper seven pairs are termed true or sternal ribs : the remainder are false or asternal, the last two ribs on each side being sometimes referred to as " floating ribs." There are sometimes eight sternal ribs.
The wall of the thorax is completed by intercostal muscles and membranes in the intercostal spaces between the ribs, and pleura hnes it on its inner side.
The ribs and sternum constitute a firm but movable thoracic cage protecting the viscera. The firmness due to the bony elements obviates collapse from atmospheric pressure, the lungs being expanded. Hence the intrathoracic pressure can be below that of the atmosphere. The power of movement of the walls is associated, of course, with respiration.